Vaillant VCW240H Combi problem - no cycling and cutting out

[dripfixer]

I've got an intermittent cut out on a VCW240H - the pilot and main burner both go out randomly, though more often when the boiler is fired up from cold.

The boiler has just recently been put back into service after 6 months off, so I've serviced it including flue check, burner cleaned, gas rate etc, replaced the thermocouple (including the in-line interrupter thermostat), spark electrode, pilot injector and pilot gas supply pipes from the gas valve to the pilot injector and from the injector to the pilot burner.

Although the boiler will fire from cold on the lowest temperature setting (1) it won't cycle if left on this setting and similarly if you start it above setting 5 and then turn it down to 1 it won't cycle.

When the boiler is running I'm getting no voltage through the overheat thermostat which I understand is correct, but 11v through the DHW stat which seems odd, though if I turn on a hot tap the boiler fires immediately.

My first thought is a faulty thermostat regulator PCB - I understand there are two PCBs on this particular boiler.

Help!

 

[Bunnyman]

These are old models now but basically it won't cycle on number 1 as this is only 35c so to low ,we were always told that 1-4 was for underfloor heating ,boiler should always b on minimum 5 an 7 in winter ,put it on 5 an it should b ok

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• Bunnyman
• 26 Mar 2010
• 499

. Ps there is no dhw stat on this model the black knob is only for ch also is there a letter x after the 240? If so the later model has a flue sensor hidden above the main ht exch this can cause probs with intermittent firing

 

[dripfixer]

Aha! I hadn't realised about the lower settings. I will make sure it's on 5 or higher.

The other problem is that it still cuts out - pilot and main burner go off and the boiler has to be restarted manually.

On the thermostats, there's an overheat stat on the pipe leading into the heat exchanger, a sensor on the underside of the boiler (the one that interrupts the cable between thermocouple and gas valve) and a CH stat next to it, plus two more sensors on what seems to be the DHW pipe at the lower left side of the boiler. The upper sensor just has one lead whilst the lower sensor has three!

There's no X in the model number on the data plate, just VCW GB 240H. The GC number is 47-044-15.

Do you know if the 'H' stands for hybrid?

Thanks for the response and any further help you can give Bunnyman.

 

[Bunnyman]

The one with the White wire on the dhw ht exch pipe is just a limit stat that interiors ntc if it overheats an all vow open flue boilers are hybrids they didn't make a non hybrid version,these models can have pilot probes if main heatexch fins are not spotless especially above where the pilot flame is they tend to get a bit of scale on the fins,is it overheating?

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• Bunnyman
• 26 Mar 2010
• 499

 

[dripfixer]

Nope, doesn't look like overheating, as it can cut out on first firing up from cold when temp gauge is reading only 20-40C.